Quantum entanglement occurs when two subatomic particles become linked in such a way that their properties remain connected, no matter how far apart they are. A change to one particle seems to instantaneously influence the other, even if they are separated by billions of light-years.
In 1964, physicist John Bell formulated what became known as Bell's Theorem, showing that if quantum mechanics is correct, such correlations cannot be explained by any hidden local variables. This idea challenged classical physics and built upon earlier doubts expressed by Albert Einstein, who famously called entanglement "spooky action at a distance." Einstein believed that no information could travel faster than the speed of light, but entanglement experiments suggest the universe doesn't always play by the classical rules.
